The present invention relates to a jet pump system for a water jet propelled boat, more particularly such a system which controls the water inlet as a function of boat speed.
Water jet propelled boats are well-known in the art and typically have a motor driven impeller located in a water duct. Water, which is drawn into the duct through a water intake opening in the bottom of the boat, is accelerated by the impeller and is ejected through a steering nozzle located in the stern of the boat. The reaction force of the water through the nozzle propels the boat forward. The nozzle may be pivoted about a generally vertical axis to steer the boat.
In the past, the water intake opening and the water duct have been made of a rigid material, such as metal or fiberglass reinforced plastic (FRP) and have been fixed in area. The fixed areas of the water intake opening and the water duct have inherently resulted in a compromise in boat performance. Depending upon the speed of the boat, different dynamic pressures act on the water intake. The dynamic pressures are higher when the boat is running at high speed and are lower when it is running at low speeds. Therefore, in boats where high speed operating characteristics are important, the water intake opening has been designed to have a relatively small area to prevent unneeded water from being introduced into the water duct which thereby increases drag on the boat. The smaller water intake opening allows the boat to achieve optimum speeds.
With the high speed boats, however, their low speed acceleration characteristics are poor. Because of the small area of the water intake opening, which facilitates high speed operation, almost no dynamic pressure is acting upon it during low speed operations. Even if the impeller can draw some water into the water duct, there is increased resistance at the water intake opening, due to its small area, which prevents sufficient water from being drawn into the water duct to achieve good acceleration characteristics.
In boats intended for low speed operation, the water intake opening is designed with a large area to enable sufficient water to be drawn into the opening with little dynamic pressure at low speeds. With this type of boat propulsion, however, the dynamic pressure increases when cruising at high speeds since more water is drawn in than is needed by the pump. This increases pump resistance and lowers the maximum speed.
Thus, the known water jet propelled boats with fixed water intake openings could not achieve both high and low speed optimum operations.
The adjustment of the water intake angle also contributes to the enhanced operational characteristics. When cruising at low speeds, the relative speed between the boat and the water is low and, in a direction parallel to the water intake opening (parallel to the bottom of the boat) there is a low water inflow speed. Therefore, a higher water entry angle at low speeds allows water to flow into the duct without significant resistance. This results in good low speed acceleration characteristics.
When operating at high speeds, however, because of the greater water entry speed in a direction parallel to the water intake opening, water becomes detached from the leading edge of the water inlet, thereby increasing the duct resistance, lowering intake efficiency and lowering maximum speed. If the water entry angle is reduced at the water intake opening, this high speed shear is prevented, thereby enhancing high speed operation. However, this increases the intake resistance at the water intake opening during low speed operation and causes poor acceleration characteristics.